Chris, thanks a lot for your answers!
Meanwhile - I investigated the current Targeted Excel version regarding the issue - Table.Buffer for different Queries (Query is a Field of #sections[Section1]). And - I found out that in the Targeted version of PQ Table.Buffer has effect even for different Queries, at least for files on a local disk.
So if you have Q1s1 = Table.Buffer(datasource) and Q1 -> Q2 -> Q4 and at the same time Q1 -> Q3 -> Q4, and refresh Q4, PQ does just 1 call to the datasource. That's important improvement (of course if I properly understood Ehren's message about
the logic of previous versions of PQ).